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A308449
Numbers k such that 25*10^(2*k) + 5*10^k + 1 are prime.
1
0, 1, 2, 14, 174, 448, 16562
OFFSET
1,3
EXAMPLE
31 is prime ==> a(1) = 0.
2551 is prime ==> a(2) = 1.
250501 is prime ==> a(3) = 2.
25005001 = 7 * 79 * 103 * 439.
2500050001 = 19 * 2269 * 57991.
250000500001 = 7 * 103561 * 344863.
PROG
(PARI) for(k=0, 1e3, if(ispseudoprime(25*100^k+5*10^k+1), print1(k", ")))
CROSSREFS
Cf. A309738.
Sequence in context: A235369 A351274 A228476 * A233224 A167014 A370909
KEYWORD
nonn,more,base
AUTHOR
Seiichi Manyama, Aug 15 2019
EXTENSIONS
a(7) from Michael S. Branicky, Apr 17 2023
STATUS
approved